![Digital Equipment Alpha 21164PC Скачать руководство пользователя страница 320](http://html.mh-extra.com/html/digital-equipment/alpha-21164pc/alpha-21164pc_hardware-reference-manual_2498508320.webp)
A–16
Alpha Instruction Set
29 September 1997 – Subject To Change
21164PC Microprocessor IEEE Floating-Point Conformance
The divide-by-zero trap is always enabled. If the trap occurs, then the destination
register is UNPREDICTABLE. For VAX architecture format, this exception is
signaled whenever the numerator is valid and the denominator is zero. For IEEE
format, this exception is signaled whenever the numerator is valid and nonzero,
with a denominator of ±0. If the exception occurs, then FPCR<DZE> is set and
the trap is signaled to the IDU.
For IEEE format divides, 0/0 signals INV, not DZE.
•
Floating overflow (OVF)
The floating overflow trap is always enabled. If the trap occurs, then the destina-
tion register is UNPREDICTABLE. The exception is signaled if the rounded
result exceeds in magnitude the largest finite number, which can be represented
by the destination format. This applies only to operations whose destination is a
floating-point data type. If the exception occurs, then FPCR<OVF> is set and the
trap is signaled to the IDU.
•
Underflow (UNF)
The underflow trap can be disabled. If underflow occurs, then the destination
register is forced to a true zero, consisting of a full 64 bits of zero. This is done
even if the proper IEEE result would have been -0. The exception is signaled if
the rounded result is smaller in magnitude than the smallest finite number that
can be represented by the destination format. If the exception occurs, then
FPCR<UNF> is set. If the trap is enabled, then the trap is signaled to the IDU.
The 21164PC never produces a denormal number; underflow occurs instead.
•
Inexact (INE)
The inexact trap can be disabled. The destination register always contains the
properly rounded result, whether the trap is enabled. The exception is signaled if
the rounded result is different from what would have been produced if infinite
precision (infinitely wide data) were available. For floating-point results, this
requires both an infinite precision exponent and fraction. For integer results, this
requires an infinite precision integer and an integral result. If the exception
occurs, then FPCR<INE> is set. If the trap is enabled, then the trap is signaled to
the IDU.
The IEEE-754 specification allows INE to occur concurrently with either OVF
or UNF. Whenever OVF is signaled (if the inexact trap is enabled), INE is also
signaled. Whenever UNF is signaled (if the inexact trap is enabled), INE is also
signaled. The inexact trap also occurs concurrently with integer overflow. All
valid opcodes that enable INE also enable both overflow and underflow.